When semiconductor elements mounted in various electronic/electric instruments such as computers, and in other heat-generating components and the like are used, the way for cooling down the heat generated in a large quantity is an issue to be investigated. In addition, as each instrument including heat generating parts such as semiconductor elements mounted therein has further reduced size, and also as the quantity of heat generation increases, the housing has become more compact, and heat generation source has been localized. In recent years, in order to deal with such localized and a large quantity of heat generation, use of a graphite film in cooling has been focused, and the demand therefore has been gradually increasing. In order to meet such demands for graphite films, investigation for production of a long and large-area graphite film using a polymer film in a state being wound to give a roll form as a raw material has been performed.
For example, Patent Document 1 discloses a method in which a long polymer film is wrapped around a cylinder and subjected to a heat treatment. Also, Patent Document 2 proposes a method in which expanded graphite and a polymer film are overlapped and wrapped around a graphitic carbon cylinder so as not to bring the polymer film layers into contact with each other, and then baked. Furthermore, Patent Document 3 proposes a method in which a material film having a width of 250 mm and a length of 30 m is wrapped around a carbonaceous cylindrical core having an external diameter of 150 mm, and is subjected to a heat treatment.
In Patent Document 1, graphitization is carried out while maintaining a state in which a film sheet is overlaid around a core. However, in this method, wrinkles and bents are generated at peripheral ends, and thus the size was limited to about 170×180 mm. Still further, when the winding number around the core is increased in this method, the film is broken due to fusion of the polymer film surfaces, and shrinkage of the polymer film resulting from the thermal degradation reaction. Furthermore, when this method is carried out in a greater scale of the size, change in dimension in graphitization leads to still larger wrinkles and bents generated at the peripheral ends.
In addition, according to Patent Document 2, graphitization is carried out while maintaining a state in which a polymer film is overlapped with filmy graphite around the core; however, the film is likely to be broken due to friction between the polymer film and the filmy graphite film according to this method, and wrinkles and bents are more likely to be generated due to restriction of the change in the dimension of the film during the heat treatment, thereby leading to a limitation of the size of about 400×1,600 mm. Moreover, an increase in the winding number around the core, or an increase in the size may result in further generation of breakages and wrinkles of the film, and thus it is difficult to obtain a long and large-area graphite film.
In Patent Document 3, graphitization is carried out while a film is wrapped around a carbon core having a diameter (φ) of not less than 120 mm; however, according to this method, graphitization of a material film having a width of 250 mm and a length of 30 m enables a long and large-area graphite film to be obtained without generation of wrinkles and/or bents which can be easily stretched to be flat by eliminating curling of the cylindrical shape without rolling. However, the resultant film is accompanied by significant ruffling, and unevenness of the thickness generated in the entirety as shown in FIG. 1.
Graphite films having ruffling as shown in FIG. 1 is very inferior in processibility and handlability. Thus, it is difficult to, for example, laminate with other sheet, and failure in winding with ends aligned occurs. Also, creases are generated even in rolling steps. In addition, when the graphite film is used as a heat radiation sheet, it fails to come into close contact well on a heat generator, and the heat spreading ability thereof cannot be sufficiently exerted. Also when the graphite film is used as a vacuum sealing, the presence of ruffling matters since it may be a factor that contributes to gas leakage.